Development of radiological/nuclear medical countermeasures to treat Acute Radiation Syndrome (ARS) is a high priority research area for NIAID and the U.S. government due to the potential threat of a terrorist nuclear attack or nuclear accident. Bone marrow is one of the most sensitive tissues to radiation damage and impaired hematopoiesis is one of the first clinical signs of high dose radiation exposure, often resulting i death within weeks due to infections or uncontrolled bleeding (referred to as the hematopoietic syndrome of the Acute Radiation Syndrome, or H-ARS). Subjects that survive the acute effects of high dose radiation on the hematopoietic system display lifelong hematopoietic defects that are believed to be due to residual damage to the hematopoietic stem cell compartment [referred to as residual bone marrow damage (RBMD) or delayed hematopoietic effects of acute radiation exposure (H-DEARE)]. G-CSF, GM- CSF and IL-11 are hematopoietic growth factors that stimulate bone marrow cells to divide and differentiate into neutrophils (G-CSF and GM-CSF) or platelets (IL-11). These proteins are used to treat chemotherapy-related neutropenia and thrombocytopenia in cancer patients. G-CSF, GM- CSF and IL-11 have short half-life in humans, which necessitates daily dosing for two weeks or more in cancer patients. Recent studies indicate that the proteins can improve overall survival in animal models of H-ARS, although, as seen in cancer chemotherapy patients, the drugs require multiple daily administrations for effectiveness. Daily dosing requires considerable healthcare provider time and may prove difficult in a mass casualty situation such as following a radiological/nuclear disaster. To reduce the need for daily dosing we developed long-acting G- CSF, GM-CSF and IL-11 analogs and showed that they significantly increase survival and accelerate multilineage hematopoietic recovery in a well-characterized mouse H-ARS model. Importantly, the proteins are effective when administered only one to three times beginning 24 h following lethal radiation exposure. The reduced dosing frequency required for these proteins will provide significant treatment advantages in a nuclear emergency setting, where healthcare provider time will be at a premium and daily dosing of patients may not be possible. This grant focuses on expanding our understanding of the utility of these proteins for treating the acute and delayed effects of radiation exposure on the hematopoietic system in adult and geriatric populations. We will determine whether there are additive or synergistic interactions of combinations of the proteins on survival and hematopoietic recovery in the mouse H-ARS model. We will explore the mechanism(s) of action of the proteins in lethally-irradiated animals by analyzing changes in hematopoietic stem and progenitor cell populations, bone marrow morphology and inflammatory status over time. We will follow surviving mice for up to two years to determine whether early treatment with the proteins or combinations of the proteins mitigates long-term damage to the hematopoietic system and hematopoietic stem cell compartment. Little is known about the radiation sensitivity and effects of radiation mitigators in older patient populations. To address this need we will develop a geriatric mouse H-ARS model that can be used to screen candidate radiation mitigators for this specialized patient population. We will determine whether long-acting G-CSF, GM-CSF and IL-11 analogs, which significantly improve survival in the adult mouse H-ARS model, also significantly improve survival in the geriatric mouse H-ARS model. These studies will provide important information about the function and mechanisms of action of G-CSF, GM-CSF and IL-11 proteins for treating the acute and delayed effects of high dose radiation exposure on the hematopoietic system, and provide a valuable screening tool for evaluating potential radiation mitigators in the important but underserved geriatric patient population.